Regulating niche of periodontium mesenchymal stem cells under the physiological condition The periodontium is composed of cementum, alveolar bone and periodontal ligament (PDL) in between. Their physiological turnover was known to be supported by stem cell populations1, 2. Based on mostly in vitro approaches, the periodontal stem cells (PDLSC) were isolated from human molar PDL3. Despite of that, in vivo location and identification of the periodontium stem cells remain largely unknown. Periodontium regeneration during or after periodontitis is a most challenging issue despite of various treatment strategies being designed. The regeneration capability difference strongly suggests that periodontium stem cells behave differentially under physiological or pathological conditions. Activity of stem cells was known to be regulated by the niche they are residing in. Various niche signals interplay which each other and keep stem cells in a dynamic balance4, 5. Despite of tremendous progress of the niche studies for other stem cell populations, the in vivo niche of periodontium stem cells has never been studied. To address above challenges, it is therefore imperative to find out the in vivo identity of the periodontium mesenchymal stem cells (MSCs) and to learn their niche organization. Based on our preliminary experiments, Gli1+ cells are identified as the MSCs for adult periodontium tissue. The Gli1+ cells are exclusively surrounding the neurovascular bundle and are more enriched in the apical region of the PDL space. These Gli1+ cells are negative for lineage differentiation or classical MSC markers. They give rise to the PDL, cementum, alveolar bone and apical root pulp during physiological turnover. Blockage of canonical Wnt signaling leads to failure of Gli1+ stem cells activation and severe periodontal tissue loss. With these preliminary findings, comprehensive investigation is proposed for investigating the in vivo properties and regulating niche of Gli1+ periodontium MSCs under physiological condition. The hypothesis is that Gli1+ MSCs are the dominant stem cell population within the periodontium and are regulated by a negative feedback loop within the periodontium. Canonical Wnt signaling pathway activates and maintains periodontium MSCs. Sclerostin ligand secreted from the cementum and alveolar bone negatively regulates the Gli1+ stem cell activities. Interplays between the two opposing signals keep the periodontium MSCs in a dynamic balance.